Convert change detection to a Python script - #129627

The change detection workflow is becoming increasingly complex.

We have grown from a fairly simple grep command to skip documentation files (#19983) to a large and complex shell script (embedded within a YAML document), indeed one sufficiently complex to merit a dedicated workflow file (#122336)

A potted history of significant revisions is thus:

• Skip CI expensive checks on CODEOWNERS update #128754
• gh-122965: Fix reusable-change-detection.yml on workflow_dispatch #122966
• Move change detection to separate workflow in CI #122336
• gh-119146: Update regexp in build.yml to not trigger the jobs on *.md and *.ini files. #120435
• gh-60283: Check for redefined test names in CI #109161
• gh-107652: Set up CIFuzz to run fuzz targets continuously #107653
• GH-86275: Implementation of hypothesis stubs for property-based tests, with zoneinfo tests #22863
• bpo-40548: Fix "Check for source changes (pull_request)" GH Action job #21806
• bpo-40548: skip build/tests on documentation changes #19983

There are further proposed changes, such as skipping Windows tests on changes to the Unix build configuration:

• Skip Windows CI for configure/Makefile changes #128446
• gh-128446: Skip Windows CI for Unix build system changes #128450

Even having recently improved readability of the central grep command (#128754), this workflow remains difficult to correctly modify (#128450).

This PR converts the core logic into a short Python script, Tools/build/compute-changes.py, which determines which workflows to run. I imagine this will make it easier to introduce future conditional workflows, which we should probably adopt more of to reduce time and resources spent waiting for CI.

I have reused the "changed files" logic introduced in #108065, meaning we can also combine the duplicative MSI and Docs changes steps, which reduces the overall work done.

I've tested this quite a bit on my fork and detection works well, as does workflow dispatch.

A

cc @webknjaz (sorry for the ping; I can't request-review)